1. Field of the Invention
This invention involves the preparation of zinc hexasulfide amine complexes.
2. Prior Art
Complexes of zinc hexasulfide with an amine are described in an article in the publication, xe2x80x9cDirect Approaches to Zinc Polychalcogenide Chemistry: ZnS6(N-MeIm)2 and ZnSe4(N-MeIm)2xe2x80x9d by Dev, Ramli, Rauchfuss and Stern, JACS 112, 6385 (1990). The article describes the preparation and properties of the complexes, and suggests that they are xe2x80x9crelevant to the action of zinc catalysts for the addition of polysulfur radicals to polyolefins in the rubber vulcanization process.xe2x80x9d
It is known to make zinc hexasulfide amine complexes (as shown in JACS 112, 6385) by reacting zinc powder, sulfur and a molar excess of the appropriate amine at an elevated temperature; then cooling the reaction product to room temperature and diluting with ethanol. The product appears, on cooling, as a yellow powder which is recovered by filtration. While the zinc hexasulfide amine complexes can be made by this general process, it is also known to produce one complex from another by amine substitution, whereby the desired amine is reacted with a completed complex, displacing the amine from the complex and substituting the desired amine thereon.
In the article xe2x80x9cDonor Solvent Mediated Reactions of Elemental Zinc and Sulfur, sans Explosionxe2x80x9d, Verma et al, Inorganic Chemistry, 1995, pages 3072-3078, there is a teaching of a reaction of zinc dust, sulfur and N-methylimidazole (Melm) at 90xc2x0 C. for a few hours. The reaction mixture is diluted with a nonpolar solvent which leads to precipitation of yellow microcrystals of ZnS6-(Melm)2. In the experimental section of the article it is mentioned that the zinc used was xe2x88x92325 mesh, which would be less than 45 microns.
It has been found that zinc hexasulfide amine complexes, as made by the process of the present invention and included in effective amounts in vulcanizable rubber, give compositions which vulcanize much faster, more quickly than the formulations previously in use. Specifically, the presence of zinc hexasulfide amine complexes in an amount of from 0.1 to 10 parts by weight per 100 parts by weight of vulcanizable rubber, proves to be effective in producing vulcanizates with fast cure rates and superior physical properties.
In one embodiment the present invention is a process for the manufacture of zinc hexasulfide amine complexes comprising reacting zinc, sulfur and a molar excess of amine at an elevated temperature to obtain a reaction mixture comprising zinc hexasulfide amine complexes and excess amine. Most of the excess amine may be separated from solid zinc hexasulfide amine complexes in the reaction mixture by techniques such as decantation, filtering and/or siphoning. A first solvent is added to the reactants or to the reaction mixture in which the zinc hexasulfide amine complexes are largely not soluble in an amount sufficient to obtain a slurry of the reaction mixture in the first solvent suitable for a subsequent separation process in which zinc hexasulfide amine complexes are to be recovered. The zinc hexasulfide amine complexes product is recovered as a solid in the subsequent separation process.
In a second embodiment the present invention is a process for the manufacture of zinc hexasulfide amine complexes comprising reacting zinc, which is at least about 99.5% pure with a particle size xe2x88x92100 mesh, with sulfur and a molar excess of amine at an elevated temperature. A reaction mixture is obtained comprising zinc hexasulfide amine complexes and excess amine. Most of the excess amine may be separated from solid zinc hexasulfide amine complexes in the reaction mixture by techniques such as decantation, filtering and/or siphoning. A solvent is added to the remaining reaction mixture in which the zinc hexasulfide amine complexes are not soluble. The zinc hexasulfide amine complexes product is recovered by filtering zinc hexasulfide amine complexes as solids from the reaction mixture and drying the solids to obtain the product.
Other embodiments of the present invention are based on details including reaction conditions, reactant and solvent compositions and process details, all of which are hereinafter disclosed in the following discussion of each of these facets of the invention.
The reaction of the process of the invention is most effectively conducted at a temperature of from about 50xc2x0 C. to about 125xc2x0 C. for about 0.5 hours to about 10 hours.
Complexes of zinc hexasulfide and an amine that are effective in the compositions of the invention include zinc hexasulfide which has been complexed with an amine of from 3 to 24 carbon atoms. Preferred amines include tertiary amines and tertiary diamines, or mixtures thereof. Among these preferred amines are N,N,Nxe2x80x2,Nxe2x80x2-tetramethylethylenediamine (TMEDA), 1,1,3,3-tetramethylguanidine, 1-Butylimidazole, 1,2-Dipiperidinoethane, 1,2-Dimorpholinoethane, 1,2-Di-(N-Methylpiperazino) ethane, N-Methylimidazole, 1-Benzyl-2-methylimidazole, 1,2-Dimethylimidazole, N-3-Aminopropylimidazole, 1-Vinylimidazole, 1-Allylimidazole, N-Cyclohexyl-Nxe2x80x2,Nxe2x80x2,Nxe2x80x3,Nxe2x80x3-tetramethylguanidine and 4-Dimethylaminopyridine, or mixtures thereof.
The preferred amine is N,N,Nxe2x80x2, Nxe2x80x2-tetramethylethylenediamine, and particularly preferred to be at least about 85% N,N,Nxe2x80x2,Nxe2x80x2-tetramethylethylenediamine and less than about 2 wt. % water.
There should be a sufficient amount of first solvent added to the reactants or reaction mixture to reduce the amount of amine required in the process to be a little or moderately in excess of the stoichiometric requirement for an effective reaction as opposed to a large excess. A preferred amount of added first solvent will reduce the amount of amine required from about 2 to about 6 times the stochiometric amount.
It would be advantageous for most of the excess amine to be separated as a supernatant from solid zinc hexasulfide amine complexes in the reaction mixture prior to addition of the first solvent by a technique such as decantation, filtering and/or siphoning. When most of the excess amine is separated from solid zinc hexasulfide amine complexes in the reaction mixture the product may be washed with the first solvent to remove excess amine and impurities.
The zinc hexasulfide amine complexes may be separated from the reaction mixture as a cake of solids, and the cake washed to minimize color and/or odor from zinc hexasulfide amine complexes product. The cake may be washed with an aqueous solution of a caustic or hypochlorite.
The amine may not be miscible with the first solvent. In that case, the reaction mixture may be filtered and the product washed with the first solvent. The filtrate would form two liquid phases comprising a first solvent phase and an amine phase, the first solvent and amine being recycled and reused in the process.
The first solvent may comprise saturated hydrocarbons, aromatic hydrocarbons or alcohols, preferably included in the group of hexanes, heptanes, toluenes, xylenes, methanol, ethanol and propanol.
The solid zinc hexasulfide amine complexes may be purified by being dissolved in a second solvent in which they are soluble, followed by separating the resulting solution from insolubles and precipitating the complexes by adding first solvent to the solution. The second solvent could comprise chloroform, methylene chloride or N-methyl-2-pyrrolidone (NMP).
It was surprisingly discovered that when at least about 99.5% pure zinc was used in the reaction of the invention, preferably with a particle size of xe2x88x92100 mesh (all particles that are less than 150 microns will fall through a 100 mesh screen), the amount of amine (TMEDA) that can be removed by decantation, filtering and/or siphoning following the reaction is significantly more than with a less pure zinc of a particle size of less than 10 microns. Even more surprising was the finding that the product had properties similar to the recrystallized product produced in the laboratory. These findings are very important, since ease of removal of excess amine would cut down the requirements of distilling amine from the amine/first solvent filtrate recovered from the product separation process in view of there being less amine in that filtrate, and a product close to the quality of recrystallized product can be obtained which results in improved performance of the crude product in rubber. Furthermore, the need to introduce exotic solvents such as NMP, chloroform and methylene chloride, which are solvents of choice for recrystallization of this product, is eliminated.
The above article from Inorganic Chemistry teaches the importance of grain size of the zinc (xe2x88x92325 mesh is the size mentioned), the smaller the grain size the better the reaction. One would therefore be led to believe by this article that it would be pointless to go to a much larger particle size, such as 100 microns or greater, because doing so would limit the reaction. One of ordinary skill in the art would thus be dissuaded by this reference from employing larger particle size zinc that would result in larger particle size product and would facilitate separation of product from the reaction mixture, because he would believe the reaction could not be made to occur.
This finding enables the use of large particle size high purity zinc, i.e. at least about 99.5% pure, to obtain a product having quality significantly better than what is obtained from less pure zinc with less than 10 micron particle size, the usual commercial grade. The performance of the product derived from 99.5% or better zinc in rubber is as good as that from recrystallized product. Thus the use of 99.5% or purer zinc is a particularly preferred embodiment of the invention.
Even more fundamental, however, is that Inorganic Chemistry teaches dilution of the cooled reaction mixture with a nonpolar solvent to recover product by precipitation. In the present invention there is no need to add any additional solvent to precipitate product, since the product is largely insoluble in the excess amine present and decantation of excess amine may be performed without addition of another solvent. The product is also largely insoluble (less than 5 wt. %)in the first solvent that is added to the reactants or reaction mixture in accordance with the present invention to obtain a slurry from which the product may be removed by simple filtration.
Whatever little solubility the product may have in the amine does not necessarily negatively impact the process, because the decanted amine may be recycled. The recycled amine would be saturated with product so there would be no significant loss in the unreacted/excess amine. The result is increased yield of product.
With regard to a general procedure for carrying out the present invention, the following non-limiting discussion is intended to provide illustrations of preferred embodiments. In a typical experiment, a reactor equipped with heating mantle, mechanical stirrer, thermocouple, condenser and a nitrogen purge line is charged with zinc, sulfur and amine (N,N,Nxe2x80x2,Nxe2x80x2-tetramethylethylenediamine-TMEDA). The best charging system has been found to be: first charging part of the amine and adding all of the sulfur while stirring. This mixture is stirred for a few minutes to make thick slurry and then zinc is added slowly while stirring so that it does not settle to the bottom of the reactor and remains dispersed throughout the reaction mixture. Finally, the rest of the amine is added to clean up the residual material in the addition funnel thereby effectively bringing all of the raw materials to the reactor.
The mixture is then heated to typically about 90xc2x0 C. for about 2 hours. During this time the reaction goes through an exotherm and the temperature could go up as high as 125xc2x0 C. The reaction mixture color goes through different shades of green. After a reaction time of about 2 hr at 90xc2x0 C., the reaction mixture is typically cooled to about 70xc2x0 C. and excess amine is decanted off using a fritted glass tube and by applying vacuum to remove liquid while leaving behind solids in the reactor. There could be some loss of material due to the solubility of product in amine, which would be more at higher temperature. Hence, cooling further down to a lower temperature and then performing decantation could potentially give higher yields due to less loss of material. Other techniques could be used to separate liquid from solids. One can filter the solids at this point and wash the solids with an appropriate solvent (typically methanol). The filtrate and washings could be kept separate and the filtrate can be recycled back. Washings containing amine and solvent can be separated by distillation or phase separation and recycled, or could be reused without further separation.
To the remaining mass in the reactor, an appropriate solvent (typically methanol) is added and the mixture is stirred before filtering the mixture to give product. The product is then washed with solvent and then dried. The product is a greenish yellow solid with melting point greater than 145-150xc2x0 C., typically 155-160xc2x0 C. The product is isolated in yields as high as 96%.
Instead of isolating the product and then recrystallizing, in the above mentioned process, after decantation of amine, one can perform recrystallization. A solvent such as N-methyl-2-pyrrolidone (NMP) or chloroform etc in which product is soluble can be added, insolubles removed via filtration or centrifugation, and then a solvent in which product is insoluble such as methanol, hexanes, heptanes etc can be added to the filtrate to crystallize/precipitate the solids.
Various process parameters have been studied.
1. Mixing rate (stirrer type and speed): Proper mixing of reactants is desired to get optimum yields of product. If mixing is not good then zinc, being heavy, will settle to the bottom of the reactor and may form a large lump that would not be reactive and in turn would give lower yields. The effect of stirring speed (RPM) on yield is evident, i.e. higher stirring will give better yields by effectively utilizing all of the zinc. Another plausible explanation is that at higher speeds zinc would not fall to the bottom, but would remain suspended in the reaction mixture.
2. Sulfur: Rubber makers sulfur may be used. Other grades of sulfur have also been successfully evaluated. The preferred sulfur level is 6 moles for every mole of zinc, however, a small excess of sulfur (2-6% in excess of the molar requirement) will give better results.
3. Zinc: In most experiments 98+% pure zinc (but less than 99.5%) from Aldrich with less than 10 micron particle size was used. In other experiments 99.9% zinc with xe2x88x92100 mesh (less than 150 micron) particle size from Johnson Matthey was used. It was surprising to see a significantly higher amount of TMEDA being decanted from the experiments when 99.9% zinc (xe2x88x92100 mesh) was used compared to 98+% grade from Aldrich. Furthermore, quality of product from this 99.9% assay zinc was significantly better than the one with 98+% zinc. This was very evident when these products were mixed in rubber and evaluated for various properties from the performance point of view. The performance of the product derived from 99.9% zinc experiment was as good as that from recrystallized product. The recrystallization step is tedious and results in significant yield loss and is economically and environmentally undesirable. Use of 99.9% pure zinc offers an option of obtaining a product which is as good as recrystallized product yet does not experience yield loss nor significantly higher cost. Also, it avoids introduction of exotic solvents such as NMP, chloroform and methylene chloride which are solvents of choice for recrystallization of this product.
Furthermore, 99.9% zinc with xe2x88x92100 mesh particle size gives a significantly higher amount of TMEDA upon decantation. The greater the amount of TMEDA during decantation, the less TMEDA that will end up with solvent and hence the lower amount of TMEDA that need be recovered by distillation. This itself offers significant cost savings from the standpoint of manufacturing cost.
4. N,N,Nxe2x80x2,Nxe2x80x2-Tetramethylethylenediamine (TMEDA): Different grades of TMEDA from different suppliers have been successfully evaluated. Several recycle studies were carried out successfully using decanted TMEDA. It was found that batches using decanted/recycled TMEDA gave significantly higher yields compared to when just fresh TMEDA was used. This could be due to the fact that the decanted TMEDA is saturated with product and when it is reused, there is no additional loss of product due to solubility in TMEDA.
5. Reaction time and temperature: Two hours at 90xc2x0 C. seems sufficient to complete the reaction. During this time the reaction goes through an exotherm and the temperature may go up as high as 123xc2x0 C. If the exotherm is controlled by effective cooling, the reaction could take longer. It was also demonstrated that lower reaction temperatures (70 to 80xc2x0 C.) can be used, however, the reaction time increases. There is no adverse effect on product quality due to lower temperature and the exotherm can be controlled better.
6. Effect of solvent: Most experiments have been carried out using excess TMEDA. In most cases the amount of TMEDA used was about 6.6 moles for every mole of zinc. When the stoichiometric amount of TMEDA was used (1 mole for every mole of zinc) along with solvent such as heptane or decane, desired product was not obtained even after reaction for several hours.
However part of the TMEDA may be replaced with methanol and product still be successfully made. In one experiment 85% by weight of normally used 6.6 moles of TMEDA per mole of zinc was employed and the rest of the weight was compensated for with methanol. For example, 40.8 g zinc, 120.2 g sulfur, 408 g TMEDA and 72 g methanol (if methanol was not used then 481 g of TMEDA would have been employed) were mixed and heated for 6 hours at 86xc2x0 C. The reaction temperature stabilized at 86xc2x0 C. and the mixture was refluxed for 6 hours, although far less than 6 hours may have been sufficient. One advantage associated with this system was that the temperature was easy to control since it would not exceed reflux temperature due to the presence of solvent. Furthermore, the most expensive raw material, namely TMEDA, was used in a smaller excess than ordinarily required.
In a similar experiment, 50% by weight of the usual 6.6 moles TMEDA per mole of zinc (resulting in use of 3.3 times the stoichiometric amount of amine) was replaced with methanol and the reaction carried out for 7 hours at 72xc2x0 C. The reaction temperature stabilized at 72xc2x0 C. while under reflux. For example, 20.0 g zinc, 60.7 g sulfur, 117.5 g TMEDA (as opposed to 235 g if solvent was not used) and 117.5 g methanol were mixed and heated for 7 hours at 72xc2x0 C. The product was obtained in 91.3% yield. Once again, similar advantages to those described above are attained with this set-up. This clearly demonstrates that a certain ratio of amine to solvent may be used successfully, so long as the amine is still in excess of stoichiometric.